A power conversion device is used as a device connected between an alternating current power system and a load, and arranged to assist a load power. This power conversion device includes a direct-current power assist device including a step-up and step-down chopper and a charge device such as an electric double layer capacitor, and an inverter device. The above-mentioned assist includes a peak cut of the electric power, a leveling of the load, an electric power storage, and compensation of an instantaneous voltage drop. A patent document 1 and so on is known as this assist.
The patent document 1 includes (is constituted by) a power buffer of an instantaneous force type which is connected to a direct-current circuit of the inverter device through a bidirectional DC/DC converter, and a power buffer of an endurance type which is connected in parallel with the power buffer of the instantaneous force type through a second bidirectional DC/DC converter. The power buffer of the instantaneous force type deals with a sudden variation of the load, and thereby a lifetime of the power buffer of the endurance type is lengthened by avoiding the sudden charge and the sudden discharge of the power buffer of the endurance type.    Patent Document 1: Japanese Patent Application Publication No. 2007-60796